The transition from dinosaurs to birds is poorly understood because of the lack of well-preserved fossils, and many scientists argue that bird-like dinosaurs appear too late in the fossil record to be the true ancestors of birds.

Top: Fossil of Anchiornis huxleyi from the province of Liaoning, China. Bottom: Artist’s rendering. (Credit: Xing Xu)

In the journal Nature this week, Xing Xu and colleagues describe an exceptionally well-preserved fossil of Anchiornis huxleyi from the province of Liaoning, China. Long feathers cover the arms and tail, but also the feet, suggesting that a four-winged stage may have existed in the transition to birds.

Anchiornis huxleyi was previously thought to be a primitive bird, but closer inspection reveals that it should be assigned to the Troodontidae — a group of dinosaurs closely related to birds.

The authors date the fossil to the earliest Late Jurassic, meaning that it is the oldest bird-like dinosaur reported so far, and older than Archaeopteryx, the earliest known bird.

They conclude that the presence of such a species at this time in the fossil record effectively disputes the argument that bird-like dinosaurs appeared too late to be the ancestors of birds.